A standard machine-tool spindle rotatable about an axis is formed centered on this axis with a large-diameter frustoconically tapered bore or seat, for example corresponding to DIN 2079. In the inner or rear end of this seat are at least two angularly equispaced gripper jaws which are carried on an axially movable actuating element and which can move radially and axially. These jaws coact with cam surfaces of the spindle such that when these jaws are retracted axially backward by the actuating element they move together and when pushed axially forward they separate.
A tool for use with such a spindle has a stem or shank that is of a shape and size complementary to the seat of the spindle. In addition the tool stem has a head formed with a shoulder the jaws can grip around so that as they pull backward they grip this head and pull the tool stem tightly into the seat. The element carrying the jaws is a rod that can be spring loaded or hydraulically or pneumatically actuated or even operated by centrifugal force when the spindle is rotated. This element also normally has an end that can abut the inner end of the tool stem when this rod and its jaws are advanced axially fully forward to knock the tool loose from its tight friction fit in the tapered seat.
In order to transmit the maximum possible torque from the spindle to the tool it is necessary to make the seat and tool stem as large in diameter as possible. When, however, a finer machining operation is being carried out, it is frequently necessary to chuck a much smaller-diameter tool in the spindle. To this end an adapter is used which at one side fits with the large-diameter seat of the spindle and on the opposite side has a small-diameter seat for the tool.
Such an adapter is described in German patent No. 3,506,901 filed 27 Feb. 1986 by G. Liebau. It has a body with a stem adapted to fit complementarily in the seat of the spindle and with a flange adapted to fit flush against the front face of the spindle. This body is formed centered on the spindle axis with a forwardly opening small-diameter tapered bore shaped, for example to ISO 50 specifications, to receive a small-diameter tool that is normally equipped immediately forward of its spindle with a quick-change collar. In addition the adapter body is provided internally with a connector rod that is at least limitedly axially displaceable in the body and that has a rear end shaped like the stem head of the large-diameter tool the spindle was built for, for instance to ISO 60 specifications. This spindle is spring-biased forward in the adapter body and carries a set of jaws that work like the jaws of the spindle but that are smaller. Thus the jaws of the spindle grip the head of the connector rod to pull it back and in turn the jaws of the connector rod grip the head of the tool and pull it back, simultaneously seating the adapter stem in the spindle bore and the tool stem in the adapter bore. Forward displacement of the spindle rod pushes the connector rod forward which in turn abuts the tool to knock it free of the adapter. In addition releasable fasteners are provided that hold the adapter body on the spindle so that the adapter can only be taken off the spindle by separate action on these fasteners.
Other systems (see German patent document No. 2,419,896 and Aachener Werkzeugmaschinen-Kolloquium 1984, 7-8 June 1984, page 164, FIG. 24) operate similarly and are particularly set up for automatic machining operations where the tools are of modular sized and are changed out by robots or the like. The use of adapters is particularly troublesome because they invariably add considerably to the length of the tool. Such extra length makes it necessary to reset the machine when switching from a large-diameter tool used without an adapter to a smaller-diameter one that must be mounted in an adapter to be chucked in the spindle.